1. Field of the Invention
This invention relates to drag type rotary drill bits. More particularly, it relates to drag type rotary drill bits having cutters mounted to a cutting end of the drill bit, each of the cutters being rotatably secured to fixtures extending from the cutting end of the rotary drill bit.
2. Description of the Prior Art
There are a number of issued patents that relate to drag type rock bits as well as rock bits that are combination drag bits and rotary cone bits.
A patent application, No. 85-27893, has been filed in Great Britain on Nov. 12, 1975 and was published on May 21, 1986. The Publication No. is 2,167,107. This application was published after the filing date of the co-pending application, Ser. No. 839,434, and, of course, before the present continuation-in-part.
The British application relies on the interaction between a cutter disc and a borehole sidewall. As the bit is driven deeper into the borehole, the downward force rotates the disc cutters independent of the rotary motion of the bit. A ratchet and pawl mechanism assures a one direction rotation of the cutter wheels.
Another example of a drag bit is disclosed in U.S. Pat. No. 4,253,533, assigned to the same assignee as the present invention. This patent teaches a diamond studded insert drag bit having a multiplicity of individual diamond insert cutter blanks fixedly inserted in the face of the bit. The diamond insert blanks are strategically positioned to maximize penetration of the bit in a borehole. The bit further includes one or more wear pads adjacent the multiplicity of diamond insert cutter blanks, the wear pads serving to channel the flow of drilling fluid or "mud" emanating from fluid passages formed in the face of the bit. The wear pads seal off a portion of the borehole bottom, thereby directing hydraulic fluid across the face and over each of the strategically positioned diamond cutter blanks. The diamond cutter blanks continuously engage the borehole bottom with the same part of their circumference.
U.S. Pat. No. 4,203,496, also assigned to the same assignee as the present invention, discloses a rotary drill bit having a main body rotating about a main axis. The bit body includes a journal pin extending downwardly therefrom, with the journal forming a second axis disposed at an angle with respect to the main axis. The journal pin has a cutter rotatably mounted thereon with the cutter having a spherical outer surface which has a plurality of inner rows of tungsten carbide mounted thereon. The gage row of the cutter is also formed on the peripheral outer surface and defines a plane intersecting the intersection of the main and second axes. The cutter gage row is formed by a plurality of inserts having synthetic diamond cutting surfaces which are facing downwardly toward the apex of the cutter in order to scrape the sides of the borehole during the downwardly traveling portion of the gage insert travel cycle. The gage row diamond inserts cut the gage of the borehole during a portion of each revolution of the rotating cone as the bit body is rotated in the borehole.
Yet another U.S. Pat. No. 4,256,191, teaches a hybrid rock bit which features an independently driven cylindrical or disc shaped rotating cutter having a multiplicity of cutter inserts mounted therein. As the body of the rock bit rotates in a borehole, the cutter cone rotatably attached thereto is independently driven by the hydraulic pressure of the fluid pumped through the center of the drill pipe. The pressurized mud is pumped through the journal and the cone, causing it to rotate independently of the drill string. The mud or fluid then passes through the cone to the outside of the rock bit body. The rotating cylindrical cutter is driven to rotate around an axis that is perpendicular to the longitudinal axis of the drill string. The rotating disc shaped cutter is driven to rotate around an axis that is skewed from the longitudinal axis of the drill string.
Another U.S. Pat. No. 3,695,370, like the 4,256,191 patent, teaches the use of an independently driven cutting element. The cylindrical grinding wheel type cutter is located on the plane of the longitudinal axis of the drill string, its axis of rotating being perpendicular to the rotational axis of the drill string. This patent claims to be an improvement over disc-type bits which are driven as the result of the drill string rotation of the bit body.
U.S. Pat. No. 1,533,286 teaches a disc-type bit wherein disc cutters are mounted to rotate about a shaft that is slightly skewed from the perpendicular to the axis of bit rotation.
U.S. Pat. No. 4,298,080 teaches the use of twin disc cutter assemblies wherein a plurality of such assemblies are on the head of the drill tool with their axis of rotation being aligned radially from the axis of rotation of the drill tool head.
U.S. Pat. No. 4,553,615 teaches the use of rotatable cutting elements in a rotary drag bit wherein the cutting force of the elements comprise an agglomerate of diamond particles.
All these prior art drill bits have failed to provide a relatively inexpensive drag bit with long lasting cutters that is not susceptible to early failure as the result of overheating or that prevent the balling of the cutters and the consequent reduction in the penetration rate.